Related Record

A related record is a record in a Related Object that is connected to the current object by way of a Lookup.

If the Lookup field is in the current record then there is exactly one target record that it "looks up" to in the Related Object. So if the current record is an Order, then there is exactly one related Customer record.

If the Lookup field is in the Related Object, then there could be many related records that look up to the current record. So if the current record is a Customer, there could be many related Order records.